These functions can be used to interrogate the status of an existing database connection object.
libpq application programmers should be careful to
maintain the PGconn
abstraction. Use the accessor
functions described below to get at the contents of PGconn
.
Reference to internal PGconn
fields using
libpq-int.h
is not recommended because they are subject to change
in the future.
The following functions return parameter values established at connection.
These values are fixed for the life of the connection. If a multi-host
connection string is used, the values of PQhost
,
PQport
, and PQpass
can change if a new connection
is established using the same PGconn
object. Other values
are fixed for the lifetime of the PGconn
object.
PQdb
Returns the database name of the connection.
char *PQdb(const PGconn *conn);
PQuser
Returns the user name of the connection.
char *PQuser(const PGconn *conn);
PQpass
Returns the password of the connection.
char *PQpass(const PGconn *conn);
PQhost
Returns the server host name of the connection.
This can be a host name, an IP address, or a directory path if the
connection is via Unix socket. (The path case can be distinguished
because it will always be an absolute path, beginning
with /
.)
char *PQhost(const PGconn *conn);
PQport
Returns the port of the connection.
char *PQport(const PGconn *conn);
PQtty
Returns the debug TTY of the connection. (This is obsolete, since the server no longer pays attention to the TTY setting, but the function remains for backward compatibility.)
char *PQtty(const PGconn *conn);
PQoptions
Returns the command-line options passed in the connection request.
char *PQoptions(const PGconn *conn);
The following functions return status data that can change as operations
are executed on the PGconn
object.
PQstatus
Returns the status of the connection.
ConnStatusType PQstatus(const PGconn *conn);
The status can be one of a number of values. However, only two of
these are seen outside of an asynchronous connection procedure:
CONNECTION_OK
and
CONNECTION_BAD
. A good connection to the database
has the status CONNECTION_OK
. A failed
connection attempt is signaled by status
CONNECTION_BAD
. Ordinarily, an OK status will
remain so until PQfinish
, but a communications
failure might result in the status changing to
CONNECTION_BAD
prematurely. In that case the
application could try to recover by calling
PQreset
.
See the entry for PQconnectStartParams
, PQconnectStart
and PQconnectPoll
with regards to other status codes that
might be returned.
PQtransactionStatus
Returns the current in-transaction status of the server.
PGTransactionStatusType PQtransactionStatus(const PGconn *conn);
The status can be PQTRANS_IDLE
(currently idle),
PQTRANS_ACTIVE
(a command is in progress),
PQTRANS_INTRANS
(idle, in a valid transaction block),
or PQTRANS_INERROR
(idle, in a failed transaction block).
PQTRANS_UNKNOWN
is reported if the connection is bad.
PQTRANS_ACTIVE
is reported only when a query
has been sent to the server and not yet completed.
PQparameterStatus
Looks up a current parameter setting of the server.
const char *PQparameterStatus(const PGconn *conn, const char *paramName);
Certain parameter values are reported by the server automatically at
connection startup or whenever their values change.
PQparameterStatus
can be used to interrogate these settings.
It returns the current value of a parameter if known, or NULL
if the parameter is not known.
Parameters reported as of the current release include
server_version
,
server_encoding
,
client_encoding
,
application_name
,
is_superuser
,
session_authorization
,
DateStyle
,
IntervalStyle
,
TimeZone
,
integer_datetimes
, and
standard_conforming_strings
.
(server_encoding
, TimeZone
, and
integer_datetimes
were not reported by releases before 8.0;
standard_conforming_strings
was not reported by releases
before 8.1;
IntervalStyle
was not reported by releases before 8.4;
application_name
was not reported by releases before 9.0.)
Note that
server_version
,
server_encoding
and
integer_datetimes
cannot change after startup.
Pre-3.0-protocol servers do not report parameter settings, but
libpq includes logic to obtain values for
server_version
and client_encoding
anyway.
Applications are encouraged to use PQparameterStatus
rather than ad hoc code to determine these values.
(Beware however that on a pre-3.0 connection, changing
client_encoding
via SET
after connection
startup will not be reflected by PQparameterStatus
.)
For server_version
, see also
PQserverVersion
, which returns the information in a
numeric form that is much easier to compare against.
If no value for standard_conforming_strings
is reported,
applications can assume it is off
, that is, backslashes
are treated as escapes in string literals. Also, the presence of
this parameter can be taken as an indication that the escape string
syntax (E'...'
) is accepted.
Although the returned pointer is declared const
, it in fact
points to mutable storage associated with the PGconn
structure.
It is unwise to assume the pointer will remain valid across queries.
PQprotocolVersion
Interrogates the frontend/backend protocol being used.
int PQprotocolVersion(const PGconn *conn);
Applications might wish to use this function to determine whether certain features are supported. Currently, the possible values are 2 (2.0 protocol), 3 (3.0 protocol), or zero (connection bad). The protocol version will not change after connection startup is complete, but it could theoretically change during a connection reset. The 3.0 protocol will normally be used when communicating with PostgreSQL 7.4 or later servers; pre-7.4 servers support only protocol 2.0. (Protocol 1.0 is obsolete and not supported by libpq.)
PQserverVersion
Returns an integer representing the server version.
int PQserverVersion(const PGconn *conn);
Applications might use this function to determine the version of the database server they are connected to. The result is formed by multiplying the server's major version number by 10000 and adding the minor version number. For example, version 10.1 will be returned as 100001, and version 11.0 will be returned as 110000. Zero is returned if the connection is bad.
Prior to major version 10, PostgreSQL used
three-part version numbers in which the first two parts together
represented the major version. For those
versions, PQserverVersion
uses two digits for each
part; for example version 9.1.5 will be returned as 90105, and
version 9.2.0 will be returned as 90200.
Therefore, for purposes of determining feature compatibility,
applications should divide the result of PQserverVersion
by 100 not 10000 to determine a logical major version number.
In all release series, only the last two digits differ between
minor releases (bug-fix releases).
PQerrorMessage
Returns the error message most recently generated by an operation on the connection.
char *PQerrorMessage(const PGconn *conn);
Nearly all libpq functions will set a message for
PQerrorMessage
if they fail. Note that by
libpq convention, a nonempty
PQerrorMessage
result can consist of multiple lines,
and will include a trailing newline. The caller should not free
the result directly. It will be freed when the associated
PGconn
handle is passed to
PQfinish
. The result string should not be
expected to remain the same across operations on the
PGconn
structure.
PQsocket
Obtains the file descriptor number of the connection socket to the server. A valid descriptor will be greater than or equal to 0; a result of -1 indicates that no server connection is currently open. (This will not change during normal operation, but could change during connection setup or reset.)
int PQsocket(const PGconn *conn);
PQbackendPID
Returns the process ID (PID) of the backend process handling this connection.
int PQbackendPID(const PGconn *conn);
The backend PID is useful for debugging
purposes and for comparison to NOTIFY
messages (which include the PID of the
notifying backend process). Note that the
PID belongs to a process executing on the
database server host, not the local host!
PQconnectionNeedsPassword
Returns true (1) if the connection authentication method required a password, but none was available. Returns false (0) if not.
int PQconnectionNeedsPassword(const PGconn *conn);
This function can be applied after a failed connection attempt to decide whether to prompt the user for a password.
PQconnectionUsedPassword
Returns true (1) if the connection authentication method used a password. Returns false (0) if not.
int PQconnectionUsedPassword(const PGconn *conn);
This function can be applied after either a failed or successful connection attempt to detect whether the server demanded a password.
The following functions return information related to SSL. This information usually doesn't change after a connection is established.
PQsslInUse
Returns true (1) if the connection uses SSL, false (0) if not.
int PQsslInUse(const PGconn *conn);
PQsslAttribute
Returns SSL-related information about the connection.
const char *PQsslAttribute(const PGconn *conn, const char *attribute_name);
The list of available attributes varies depending on the SSL library being used, and the type of connection. If an attribute is not available, returns NULL.
The following attributes are commonly available:
library
Name of the SSL implementation in use. (Currently, only
"OpenSSL"
is implemented)
protocol
SSL/TLS version in use. Common values
are "TLSv1"
, "TLSv1.1"
and "TLSv1.2"
, but an implementation may
return other strings if some other protocol is used.
key_bits
Number of key bits used by the encryption algorithm.
cipher
A short name of the ciphersuite used, e.g.
"DHE-RSA-DES-CBC3-SHA"
. The names are specific
to each SSL implementation.
compression
If SSL compression is in use, returns the name of the compression algorithm, or "on" if compression is used but the algorithm is not known. If compression is not in use, returns "off".
PQsslAttributeNames
Return an array of SSL attribute names available. The array is terminated by a NULL pointer.
const char * const * PQsslAttributeNames(const PGconn *conn);
PQsslStruct
Return a pointer to an SSL-implementation-specific object describing the connection.
void *PQsslStruct(const PGconn *conn, const char *struct_name);
The struct(s) available depend on the SSL implementation in use.
For OpenSSL, there is one struct, available under the name "OpenSSL",
and it returns a pointer to the OpenSSL SSL
struct.
To use this function, code along the following lines could be used:
#include <libpq-fe.h> #include <openssl/ssl.h> ... SSL *ssl; dbconn = PQconnectdb(...); ... ssl = PQsslStruct(dbconn, "OpenSSL"); if (ssl) { /* use OpenSSL functions to access ssl */ }
This structure can be used to verify encryption levels, check server certificates, and more. Refer to the OpenSSL documentation for information about this structure.
PQgetssl
Returns the SSL structure used in the connection, or null if SSL is not in use.
void *PQgetssl(const PGconn *conn);
This function is equivalent to PQsslStruct(conn, "OpenSSL")
. It should
not be used in new applications, because the returned struct is
specific to OpenSSL and will not be available if another SSL
implementation is used. To check if a connection uses SSL, call
PQsslInUse
instead, and for more details about the
connection, use PQsslAttribute
.